BY B.Nagendra Prasad

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Outline of Presentation

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What is VCR engines?

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It is the technology to adjust internal combustion engine cylinder compression ratio.

In a VCR engine high compression ratio is employed for greater efficiency and low load operation.

Low compression ratio is employed at full load allowing the turbo charger to work without problem of detonation.

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SAAB Automobile Introduced variable compression to the world at the Geneva motor show in 2000.

Fuel consumption is 30 percent lower than conventional engine by achieving variation of compression ratio from 8:1 to 16:1 depending on the load.

VCR Engines reduces combustion chamber temperature and increases charge and thermal efficiency.

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Methods to obtainVCR

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2.Variation of combustion chamber volume

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The volume of combustion chamber is changed to obtain the variable compression ratio by moving a small secondary piston which communicates with the chamber.

The piston could be maintained at an intermediate position corresponding to the optimum compression ratio for a particular condition.

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3.Variable height piston

Variation on compression height of the piston offers potentially the most attractive route to the production of VCR engine.

But it requires relatively minor changes to the base engine architecture when compared to other options.

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4.Movement of the crankshaft or crankpins

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5.Multilink mechanismsA popular approach has been to

replace the connecting rod with a two piece design in which an upper member connects with the piston while a lower member connects with the crankshaft.

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Experimental Setup

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The connecting rod is made of three links.

Link 1 connects the piston to the intermediate link 2, which in turn is fixed.

Now the power transmitting link to the crank shaft is link 3, which joins the link 2 to crank shaft.

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Working of VCR engine When the piston is moving to the bottom dead center the

link 1 pushes the link 2 directly, thus the crank shaft also moving thus producing no dwell at the bottom dead center.

when the piston is moving towards the top dead center the link 3 will continue to be in motion but keeping the link 1 in a constant, thus giving a constant injection without stopping the crank shaft.

As main link 1 will dwell at the top dead center. The dwell that is required at the top dead center will depend on the positioning and the length of the links.

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ConclusionsThe newly developed multi link mechanism enables optimum compression ratio control such as the fuel economy (thermal efficiency) of a engine.

With the multilink VCR mechanism, the piston's reciprocal action achieves ideal simple harmonic motion. This enable low vibration property of in-line 4 cylinder engine.

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